The present invention relates to a power distributor unit particularly for agricultural, industrial and similar machines.
As is known, agricultural and industrial machines use gearboxes which try to approximate the theoretical stepless gearbox with the possibility of presetting the gear ratio and with high efficiencies.
Gearboxes currently use transmissions provided with torque converters and with a large number of power-shiftable speeds, technically known as torque converter power-shift transmissions. Occasionally transmissions are used without a torque converter, in agricultural machinery wherein efficiency is essential, or in machines for industrial use with hydrostatic motors and pumps, and having a small nunsher of power-shiftable gears arranged downstream of the hydrostatic motors.
Attempts have been made to obviate the limits of such known structures with power distributor units. Studies conducted by Kutzbach in 1929 were applied in 1938 by Fottinger with a hydraulic coupling from which, in the course of time, gearboxes known as Hydromatik, Dodge, Diwabus, Ilomatik, SRM and others available in the literature have been developed.
The use of torque converters or couplings entails disadvantages regarding their use: it is in fact not possible to control the gear ratio and to achieve high efficiency.
One solution to the above described problems was disclosed in Italian patent application no. MI91A000538 dated Mar. 1, 1991, which described a continuously-variable transmission of the type which distributes accumulated power, and which used a metallic chain acting between conical pulleys. However, it should be noted that in said patent application there is still only one power distribution step and that in order to increase the range of the continuously-variable transmission, hereinafter termed CVT, it is necessary to insert, downstream of said CVT, a two-speed unit which has a high ratio and requires power-shifting but has great difficulties in the adjustment of the metallic chain simultaneously with the clutches arranged thereat.
In order to clarify the typical requirements of a gearbox for agricultural use, it should be mentioned, by way of example, that a tractor works in two or more separate steps: as power source on the field, at speeds comprised between 2.5 and 9.5 kilometers per hour, and during transport on roads, with speeds which correspond to the maximum-power condition, equal to 10-38 kilometers per hour. The conversion ratio required in the two situations is equal to approximately 3.8 and can be used in the various steps by means of a set of gears arranged downstream and equipped with a reverse gear, possibly built into the rear axle.